In recent years, users of global positioning systems (GPS) have enjoyed real-time three-dimensional navigation capabilities at previously-unavailable performance levels. Except for their susceptibility to interference and jamming, such GPS systems have largely fulfilled the promise of accurate, worldwide satellite navigation. Nevertheless, users continue to demand increasingly high navigation performance, particularly with respect to accuracy and integrity—i.e., the ability of a navigation system to detect false navigation information.
Unfortunately, the presence of jamming and/or interference has prevented full reliance on GPS as means of navigation, especially in certain military or safety-of-life applications. In civil aviation, safety risk due to jamming is generally not an issue. Various contingency procedures have been developed to safely return aircraft to the ground at alternate airfields. However, a dependence on GPS as currently structured could result in a risk of large-scale disruption to air traffic and, therefore, commerce in general. Furthermore, the increasing extent to which GPS is embedded in day-to-day infrastructure, such as ground and marine transportation, and the timing of electrical power distribution, the Internet, cellular telephones, and financial transactions, serves to increase potential societal vulnerabilities due to intentional disruption of the GPS signal from jamming.
Because a GPS signal is relatively weak (a user receives roughly −160 dBW at the terminals of an omnidirectional antenna), it takes very little jamming to bring down navigational capability. A low-cost 5W jammer, for example, is sufficient to disrupt GPS use at a radius of several tens of miles—especially if there is line-of-sight contact with the user. Such sensitivity tends to work against the practicality of satellite navigation and, conversely, in favor of traditional higher-power navigation aids used for aviation, some of which predate GPS, including VOR, DME, ILS, TACAN, and LORAN-C.
Currently, aircraft can only use GPS for supplemental-means navigation. Traditional navigation aids are sufficient for ordinary operations and have power levels that are sufficient to resist jammers who might be tempted to disrupt commerce. Therefore, because of the signal vulnerability of GPS, there is little incentive to take advantage of the significant performance and cost advantages of satellite navigation. The FAA's Wide Area Augmentation System (WAAS) and Local Area Augmentation System (LAAS) offer the potential to bring low-cost aircraft landing capability to thousands of airports nationwide where it was never available before. Today, the United States is paying for two civil safety-of-life navigation systems: the traditional ground-based system, and the newer, more capable satellite-based system.
A number of prior art techniques are available to combat jamming. These methods focus on (i) specialized satellite design and (ii) receiver design. In the satellite, for example, it is always possible to simply increase the raw power broadcast to the ground. However, there is a price for raw power: each Watt of extra power scales up the satellite payload and launch costs accordingly, such that significant increases in broadcast power quickly become expensive. Another approach is to use wider bandwidth broadcasts that can enable additional processing gain. Here, too, there is a price to pay: efficient use of finite spectrum for multiple purposes requires significant global coordination. GPS has specific broadcast spectrum assigned to it, and it is unlikely that any new spectrum will be assigned any time in the foreseeable future.
Receiver approaches to the problem of jamming are generally divided into three categories: (i) antenna pattern shaping, (ii) signal excision, and (iii) averaging. Antenna pattern shaping uses adaptive multi-element arrays of antennas called a Controlled Radiation Pattern Antenna (CRPA) to electronically point a beam directly at a satellite and, therefore, exclude a jammer. A CRPA can also point a null at an estimated jammer direction. CRPAs can be quite effective in most circumstances, although they are generally expensive and bulky. They also have a drawback of becoming less effective when a jammer line-of-sight happens to be nearly coincident with a satellite or, worse yet, when several distributed jammers are used. In this case, the laws of physics place mathematical constraints on the number and quality of beams and nulls that can be applied to a set of jammers for a given CRPA design.
Excision refers to wide-band, pre-correlation signal processing carried out in a GPS receiver. Because the signal characteristics of GPS are well-known, any excess power due to jamming is directly observable by the receiver in real time and can be excised via notch filters, pulse blanking, or any number of other more elaborate techniques. Excision is an effective and inexpensive signal processing step and should generally be carried out as a matter of good practice. However, it is insufficient in and of itself to eliminate all the effects of interference or jamming. For example, if a jammer is broadband noise, the receiver would detect the presence of jamming, but would be unable to apply excision to selectively remove any part of it without a priori knowledge of its character. Current signal processing techniques known as Space Time Adaptive Processing (STAP) and Space Frequency Adaptive Processing (SFAP) combine the CRPA and excision into one processing stage.
Averaging techniques aim to filter out as much jamming as possible during the pre-detection interval (PDI) of the receiver. The most basic form of averaging is the processing gain provided by the ratio of the pre-correlation bandwidth (20.46 MHz) to the pre-detection bandwidth of the receiver (typically 50 Hz). For a P(Y) code receiver, averaging provides a basic level of 56 dB of jamming immunity, and here only for very low dynamics. Attempts to improve upon this level of protection have traditionally encountered several barriers to practical implementation. The first barrier comes from the 50 bps data modulation superimposed on the GPS carrier. This modulation effectively limits the PDI to 20 ms.
Data stripping is one method used to try to get around the 20 ms PDI limitation. Since the GPS broadcast message changes infrequently or in a predictable way, it is often possible to apply pre-recorded frames to remove most of the data modulation. Unfortunately, for military or safety-of-life applications, the method cannot always be counted on because the pre-recorded data message does not always track the actual broadcast message. Consistency between the two data streams can be thrown off by any number of factors, including new ephemeris uploads, operational errors, and system failures. Any inconsistency does not contribute to graceful degradation. A key improvement to the data stripping approach teaches how Low Earth Orbiting (LEO) satellites can provide a global feed forward of the GPS data bits so as to eliminate any gaps in operation. See, e.g., U.S. patent application Ser. No. 10/873,581, entitled “Real Time Data Aiding for Enhanced GPS Performance,” filed Jun. 22, 2004.
Unfortunately, regardless of whether data is removed from the GPS carrier, significant obstacles remain in attempting to narrow the pre-detection bandwidth or making use of low signal level measurements. GPS signals are made up of multiple components, including a PRN code modulation and a carrier frequency. In the absence of interference or jamming, receivers typically track both the code and the carrier. In the event of jamming, most military receivers drop out of carrier track and revert to a form of code-only tracking, wherein the raw 20 ms pre-detection samples are multiplied together using variations of a dot-product discriminator. The dot product discriminator is generally considered to be among the most effective of the squaring-type discriminators. These samples are averaged together over an extended interval—sometimes several tens of seconds—to resolve a code tracking error. The commonly-applied benefit of dot-product-type code tracking is that it has somewhat higher jam resistance than carrier tracking alone. The idea is to use an Inertial Navigation System (INS) to subtract out user dynamics, thereby permitting the noisy post-detection samples to be averaged over a long interval. The most integrated version of code-based anti-jam tracking is called “Ultra-Tight Coupling” (UTC).
Unfortunately, ultra-tightly-coupled inertials have only been effective to a certain level of protection. The physics of such systems quickly limits their ability to withstand significant jamming. First, due to squaring losses stemming from the discriminator, long integration times are required. The integration time is proportional to the square of J/S. This means that for every doubling of jamming power, the required integration interval must quadruple. Second, inertial instruments exhibit errors that grow with time. Although some inertial instruments can provide better performance at increased cost, there are practical physical limits as to how long an inertial can remove the platform dynamics without an update from GPS. This limit is usually set by the time the inertial noise takes to reach a large fraction of a code chip—usually about 5 m. For a given quality of inertial, the dependence on GPS code modulation yields a certain jamming level at which the ranging error exceeds a threshold during integration and the system is no longer useful.
Assuming that the GPS data modulation can be removed from the carrier in a dependable way, coherent tracking of the carrier has sometimes been considered but summarily dismissed as an option for increased jam immunity. Such an approach has traditionally been seen as impractical because the receiver must integrate the carrier over a sustained interval to a stability of less than 30 picoseconds (the amount of time it takes light to traverse 1 cm). The challenge is to maintain this required stability over an interval that is much longer than 20 ms. A typical low-cost Temperature Compensated quartz Crystal Oscillator (TCXO) is the basis for the vast majority of GPS receivers today. The part cost generally ranges between $10 and $20. With a TCXO, the pre-detection interval may be safely extended to a large fraction of a second. Beyond this, a TCXO is not sufficiently stable.
Other more stable exotic clocks such as ovenized quartz or atomic clocks based on rubidium or cesium frequency standards are candidates, but even these highly stable clocks have practical issues that do not render them practical. For 30 dB of additional GPS jam protection, a user needs to integrate in the neighborhood 20 seconds. At this level, even many atomic clocks are not capable of providing the needed stability. Vibration, bulk, and cost can become prohibitive. A promising new Chip Scale Atomic Clock (CSOC) approach offers potential to reduce cost, size, weight, and power some years from now, but even the most optimistic projections of performance do not achieve sufficient frequency stability to yield the required phase stability over the needed interval. Some Oven-Controlled Crystal Oscillators (OCXO) possess the required phase stability over the needed interval. However, an OCXO is typically bulky, expensive, and power consuming. A solution depending on such highly stable clocks is not readily accessible without the cost, size, weight, and power consumption associated with precise temperature control. Such a solution with high premium on component performance sensitivity is a significant technical challenge. Power, vibration, and cost become major obstacles. What is needed is a solution that could provide significantly enhanced performance using a standard, low-cost TCXO.
The Military, Civil, and Commercial sectors each have their own issues and work-arounds to jamming. The Military is perhaps best prepared to combat jamming because it is generally less cost constrained and has access to more advanced technology. Unfortunately, even relatively low-power jammers are capable of bringing down user equipment within line of sight of the jammer. In the user equipment, a broad spectrum of anti-jam capabilities are employed, often as a combination of techniques, including CRPAs and ultra-tight inertial coupling. The Military also proposes to implement a new higher-power M-code signal that is intended to boost signal power by approximately 20 dB. Large-aperture spot beam antennas would focus a tighter beam on specific regions of the Earth to concentrate more signal power there. However, even if the cost of deploying such a high-power system were not an object, it will still be many years before such a system will be available for use. What is needed is a low-cost, immediately-available navigation solution.
These military solutions, taken in the aggregate, appear to provide reasonable protection against many jamming threats predicted in the near future. However, these solutions may also fall short under future jamming scenarios—especially as mentioned previously with respect to large numbers of low-power, distributed jammers. Perhaps most important, the set of current solutions described above all tend to be expensive.
Civil vulnerability is a significant challenge. As mentioned previously, GPS already has means to counteract unintentional interference with the addition of the second civil frequency. Because only one frequency is required for many operations, if one frequency is down due to unintentional interference, the other stands a high probability of being operational. For either unintentional or intentional interference, as a last resort, an aircraft can divert to an alternate airport.
The problem of intentional jamming is much worse. Again, the objective is to deny jammers a systematic means of disrupting air travel that would interfere with the daily flow of commerce. The commercial nature of civil aviation requires that any solution to the problem of intentional jamming be cost-effective. Installing expensive user equipment adapted from the Military, such as CRPA antennas, to the civil aircraft fleet has been viewed negatively. So far, the only viable solution has been to maintain the existing navigation aids in service, such as VOR, DME, and ILS, which operate at higher power. Because satellite solutions such as the WAAS do not provide any additional value to aviation users because the existing ground aids are also in operation, there is little incentive for the airlines to transition onto satellite navigation.
Commercial users also have a stake in a non-jammed signal. In addition to the growing dependence on GPS for a variety of commercial functions in society, including timing of the Internet, the power grid, cellular telephone networks, and financial transaction timing, there is also a potential regulatory threat to GPS signal strength that could originate from Ultra-Wide Band (UWB) technology. While UWB has significant promise, there is a distinct possibility of interference if the GPS band is not carefully protected from a regulatory standpoint. Given that regulations can sometimes take time to arrive at a satisfactory balance, it would be desirable to have access to an economical technical “safety net” that would allow users to protect their investment in critical GPS-based infrastructure during this critical transition to coexistence with UWB devices.
In summary, existing systems and methods for providing jam-immunity are unsatisfactory. What is needed is a navigation system that provides high accuracy and integrity for navigation in the presence of interference and/or jamming, thereby ensuring significant and effective anti-jam protection in the near-term for a wide variety of GPS and satellite navigation applications, including military, civil, and commercial.